This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The rapidity with which P suppresses daytime LH (and by inference GnRH) pulse frequency is unknown. We propose to assess this further using a randomized, cross-over, placebo-controlled study. Ovulatory women pretreated with E2 will undergo a 24-h sampling study in the GCRC. After 10 h of sampling, either oral micronized progesterone (100 mg p.o.) suspension or placebo suspension will be administered (according to randomization). During a subsequent menstrual cycle, subjects will undergo another GCRC study identical to the first (including pretreatment with E2) except that oral progesterone will be exchanged for placebo or vice versa in accordance with the crossover design. We will assess the acute effects of progesterone on LH frequency, with secondary endpoints being mean LH, LH pulse amplitude, and mean FSH. We hypothesize that administration of P (at 0600 h) to adult women during the follicular phase will result in a demonstrable suppression of daytime LH (and by inference GnRH) pulse frequency within 12 hours.